The present invention relates to an optical single-sideband modulated signal generator using a phase shifting method and, more particularly, to an optical single-sideband modulated signal generator which effectively suppresses the upper or lower sideband of an optical double-sideband signal, generated by amplitude-modulation of an optical signal, to derive therefrom an optical single-sideband modulated signal.
In this kind of technical field, it is conventional to use such a system as shown in FIG. 21, wherein an electric modulation signal 100 for the amplitude-modulation of an optical signal is phase-shifted by a 90° phase shifter 101 in an electric signal stage, then the original modulation signal 100 and the phase-shifted modulation signal 102 are used to amplitude-modulated optical signals in separate optical amplitude modulators 103 and 104, and the thus amplitude-modulated optical signals are combined by a combiner 105 into an optical single-sideband modulated signal 106. Incidentally, an optical carrier 107 is branched by a two-output waveguide branching unit 108 into a first waveguide path 109 and a second waveguide path 110. The optical carrier 107 branched to the first waveguide path 109 is applied via an optical 90° phase shifter 111 to the optical amplitude modulator 104. The optical carrier 107 branched to the second waveguide path 110 is fed directly to the optical amplitude modulator 103.
This system can be taken as merely using an optical as an electric carrier signal in a generally known single-sideband signal generating system based on the phase shift scheme.
Since the phase shifting of the modulation signal is carried out at the electric signal stage in the above-mentioned prior art, such prior art is defective in that when the modulation signal is a high-frequency signal having many frequencies, an analog phase shifter is difficult to operate stably, whereas a digital phase shifter lacks operation speed.